Method of producing light-conducting glass structure

ABSTRACT

By heat treating a glass member containing at least one kind of cation to constitute a modifying oxide in contact with a source of another kind of cation to cause ion substitution, a lightconducting glass structure can be produced to have a refractive index distribution wherein the index varies progressively transversely to the intended light path, which is thereby bent toward the direction of increase of the index, such a lightconducting glass structure is not accompanied by differences or lagging of phase velocities of conducted light rays, spreading of the light flux width, and reflection losses.

3H m2. avazsmee L V I 1 3 w 1 it} United S '3 @1711 1111 3,923,486

Kitano et al. 1 1 Dec. 2, 1975 METHOD OF PRODUCING 3.434.774 3/1969Miller 350/96 wo L]GHT CONDUCT]NG GLASS STRUCTURE 3,486,808 12/1969Hamblen 350/175 ON [7 l I h K t K b K K $563,057 2/1971 Rosenbauer 65/305 nventors: c no I ano, o e; en oizumi,

Itami; Hiroyoshi Matsumura, Osaka, OTHER PUBLICATIONS a" f Japan Miller,Article in Bell System Technical Journal, Vol. 44, No. 9, Nov.1965,PP.20172030. [731 Asslgnee: Nihon ltagarasu Kabushiki Kalsha,

Japan Primary E.\'aminerRobert L. Lindsay, Jr. 22 F s 19 1973 Almrney,Agent, or Firm-Robert E. Burns;

Emmanuel J. Lobato; Bruce L. Adams [211 App]. No.: 398,832

Related US. Application Data [57] ABSTRACT [60] Division of $61. No.147,256. May 26, 1971, which is By heat treating glass member Containingat least a continuation of Ser, No, 806,368, March 12, 9 9 one kind ofcation to constitute a modifying oxide in abandoned. contact with asource of another kind of cation to cause ion substitution, alight-conducting glass struc- [52] US. Cl. 65/30 E; 65/DlG. 7; 350/96WG; ture can be produced to have a refractive index distri- 350/ GNbution wherein the index varies progressively trans- [51] Int. Cl. C03C15/00; G02B 5/14 versely to the intended light path, which is thereby[58] Field of Search 65/4, 30, DIG. 7, 30 E; bent toward the directionof increase of the index,

350/96 WG, 175 GN such a light-conducting glass structure is notaccompanied by differences or lagging of phase velocities of [56]References Cited conducted light rays, spreading of the light fluxwidth,

UNITED STATES PATENTS and reflection losses- 3,320,114 5/1967 Schulz65/30 X 10 Claims, 22 Drawing Figures 3923486 OR IN 65/305;

US. Patent Dec. 2, 1975 Sheet 1 of5 3,923,486

FIG. 2

2 O i TL IONS F/a. 4 E UJ Z O o r Z O E 5 IONS U 2 O o I I I DEPTH FROMSURFACE (microns) US Patent Dec. 2, 1975 Sheet 2 of5 3,923,486

0 CENTER DEPTH (microns) 0 SURFACE (microns) FIG. 3

CENTER m A mm 7 F EE 6 CR I mum mRN F mwm O XmQ2 m ru mu wm T 6 E C -0 A5 F R U :m S M w 0 R F -0 2 H T '0 P l E D O m o w w a (microns) FIG. .5

US. Patent Dec, 2, 1975 Sheet 3 of5 3,923,486

FIG. 8

R E T N a -w\ 3 0 5 2 E -OC A F 0W S M O m F H P E D E C m R U 5(microns) zocaihzwuzoo All DEPTH FROM SURFACE (mlcrons) ls'o DEPTH FROMSURFACE (microns) US. Patent CONCENTRATION (arbitrary scale) REFRACTIVEINDEX Dec. 2, 1975 CONCE NTRATION INTERFACE DISTANCE FROM CENTER OINTERFACE DISTANCE FROM CENTER F/G. IZ/ I FIG. /3

Sheet 4 of 5 (arbitrary scaleI REFRACTIVE INDEX 0 INTERFACE DISTANCEFROM CENTER FIG. (b)

I I I o \INTERFACE DISTANCE FROM CENTER Patent Sheet 5 of 5 f a lsl lszl5|{ l5lc I We KW l5|d n: o 0.05 f 24 2l l3 Zia DISTANCE FROM 239 a, If

CENTER (cm) 245257\L{ \VQZLQLB FIG. I8

FIG (9 l.6l-

E 1.59- E E F I6. 20

l.57 I I 0 50 I00 8 cENTER DEPTH FROM SURFACE (micron METHOD OFPRODUCING LIGHT-CONDUCTING GLASS STRUCTURE This is a division, ofapplication Ser. No. 147,256, filed May 26, 1971 which is a continuationof application Ser. No. 806,368 filed Mar. 12, 1969, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to light-conductingglass structures having a refractive index gradient in a directiontransverse to the direction in which light propagates therethrough andmore particularly to a method for producing these light-conducting glassstructures.

A light-conducting fibre of known type consists essentially of a corestructure of a light-conducting substance of relatively high refractiveindex and a cladding layer of a light-conducting substance of relativelylow refractive index covering the core structure. A flux of lightincident on one end of the fibre. is propagated therealong by beingrepeatedly reflected from the interface between the core structure andthe cladding layer as described hereinafter and illustrated in FIG. 1 ofthe accompanying drawings.

However, a clad-type light-conducting fibre of this known character inwhich reflection is utilized is accompanied by the following problems.First, as the light flux, the amplitude of which is varying at a veryhigh speed, and which has entered the fibre, advances by undergoingrepeated total reflection, differences in the light path lengths of theindividual light rays of the light flux occur, and, consequently,staggering or differences in phase are produced when the light fluxexits from the fibre. When such differences in the light flux phase areexistent, it is difficult to utilize the light-conducting fibre as apath for transmitting in a communication system, in which light signalsvarying at ultra high speed are employed.

Furthermore, as the incident light flux advances as it is reflected bycurved interface surfaces within the fibre, the width of the light fluxunavoidably and progressively increases, and, at the same time, therearise reflection losses at the aforementioned interface. These phenomenaare also causes of lowering of the efficiency of photocommunication andother light-transmitting systems.

A light-conducting glass fibre in which the refractive index increasesprogressively from the surface toward the interior has already beenproposed. The utilization of a glass in which the refractive indexdecreases in proportion to the square of the distance from thecentreline for a light-conducting path or a part thereof has beenproposed in order to overcome the above defects. Proceedings of theIEEE, Vol. 53, P. 2148-2149, December, 1965, and Uchida Teiji: DenshiTsushin Gakkai Soritsu SO-ShiI-Nen El en Zenkoku Taikai Symposium,YokcFShE Laser Oyo" Hen, p. 3-4, October, 1967 (translation: (Japan)Society of Electronic Communication (Engineers) Fiftieth AnniversaryNational Convention Symposium Preprint Laser Application edition, p.3-4, October, 1967) However, light-conducting glass structures orlightconducting glass fibres having such refractive index distributionscould not be realized on a commercial scale because methods of producingsuch structures have heretofore been unknown.

Furthermore, so-called gas lenses" have heretofore been known. Forexample, on pages 465, 466, and 467 of The Bell System TechnicalJournal, March Issue, 1965, it is disclosed that glass structures andother transparent structures having refractive index distributions whichdecrease or increase as the square of the distance from their centrelines exhibit a lens effect. However, structures which are transparentlens structures having such a refractive index distribution and,moreover, are practical have not heretofore been realized.

We have discovered that a glass structure having a progressively varyingrefractive index can be produced by causing the concentration of certainingredients of the glass to vary with position as described fullyhereinafter.

SUMMARY OF THE INVENTION Accordingly, it is an object of the presentinvention to utilize this finding and other findings to providelight-conducting glass structures each having a refractive indexgradient in a direction transverse to the direction in which light is toadvance.

Another object of the invention is to provide a lightconducting glassfibre or glass rod in which differences in phases of a light flux, whichvary at ultra high speed, at the outlet end, spreading of the light fluxwidth, and reflection losses are prevented, and which thereby can beutilized effectively for photocommunication.

Still another object of the invention is to provide a method forproducing light-conducting glass structures of the above statedcharacter.

A further object of the invention is to provide a method for producinglight-conducting glass structures of the above stated character which isnot accompanied by difficulties such as the formation of cracks on thesurface of the product.

Other objects of the invention will presently become apparent from thefollowing description.

The foregoing objects have been achieved by the present invention, inone aspect of which there is provided a 1i ght-conducting glassstructure in which the refractive index of the glass varies withposition by causing the concentration distributions of at least twocations constituting modifying oxides within the glass to vary withposition.

Such a light-conducting glass structure according to the invention canbe produced by a method also of the invention in which the cationconcentration distributions are varied by a process comprising ionsubstitution due to heat diffusion.

According to the present invention in one aspect thereof, brieflysummarized, there is provided a lightconducting glass structurecontaining the least two kinds of cations constituting modifying oxideswithin the glass and having therewithin a distribution of refractiveindex such that the index varies with position in a direction transverseto the direction of light conduction, which is thereby bent toward thedirection of increase of the index. This glass structure comprises glassforming oxides and modifying oxides, and the concentration of thecations within the glass varies in the direction in which the refractiveindex is to vary, the concentrations of some cations and those of othercations varying in opposite directions thereby to create refractiveindex variation.

According to the present invention in another aspect thereof, there isprovided a method for producing lightconducting glass structures of theabove stated character which method is characterised by the process ofplacing a glass material containing the aforementioned cations incontact with a source of other cations capable of constituting modifyingoxides and heat treating the glass material and cation source at atemperature permitting the cations to migrate within the glass therebyto cause at least one kind of cation located in positions nearer to theglass surface to be substituted to a greater degree by the other cation.

The nature, principles, details, and utility of the invention will bemore clearly apparent from the following detailed description, beginningwith general considerations and concluding with specific examples ofpreferred embodiment of the invention, when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is an enlarged side view, in longitudinal section (taken along aplane parallel to the light path), indicating schematically theprinciple of the light propagation through and within a light-conductingglass structure known heretofore;

FIG. 2 is a view similar to FIG. 1 indicating schematically theprinciple of light propagation through and within one example of alight-conducting glass structure according to the invention;

FIGS. 3 through 6, inclusive, are graphical representations indicatingthe characteristics of a specific example of a light-conducting glassstructure according to the invention;

FIGS. 7 and 8 are graphical representations indicating the refractiveindex distribution within a specific example of a glass structure of theinvention;

FIGS. 9 and 10 are graphical representations respectively indicatingdistributions of ion concentrations within the glass structure indicatedin FIG. 8;

FIGS. 11(a) and 11(b) are graphical representations indicating theconcentration distributions of ions constituting a glass structure inradial directions from the centre in cross sections of the structure;

FIGS. 12(a) and 12(b) are, graphical representations respectivelyindicating refractive index distributions in the same directions;

FIG. 13 is a schematic elevation, in vertical section, indicating a modeof production of a glass structure depending on the rod method andconstituting a specific example of practice of the invention;

FIG. 14 is a similar schematic elevation indicating a mode of productionof a glass structure of the invention depending on the pot method;

FIG. 15 is cross section of a multiplex light-conducting structureaccording to the invention containing two or more light-conductingmembers;

FIGS. 16 and 17 are schematic elevations, in vertical section,indicating modes of producing the glass structure illustrated in FIG.15;

FIG. 18 is a graphical representation indicating the refractive indexdistribution in a radial direction from the centre in a cross section ofa specific example of a glass structure of the invention;

FIG. 19 is a schematic elevation, in vertical section, indicating aspecific example of method for producing a glass structure according tothe invention; and

FIG. 20 is a graphical representation indicating the refractive indexdistribution within a specific example of a glass structure of theinvention.

DETAILED DESCRIPTION As conductive to a full understanding of thepresent invention, a brief consideration of the aforementionedlight-conducting fibre of heretofore known type will be first presentedwith reference to FIG. 1.

As shown in FIG. 1, this fibre consists essentially of a core structure11 of a light-transmitting substance of relatively high refractive indexand cladding layers 12a and 12b of a light-transmitting substance ofrelatively low refractive index, interfaces being formed between thecore structure and the cladding layers. In the operational use of thisfibre, an incident light flux 13 is caused to enter one end of the corestructure so as to advance with an angle greater than the reflectioncritical angle of this interfaces and is thereby reflected repeatedly bythe interfaces to be propagated through and along the fibre corestructure 11. A clad type lightconducting fibre of this character,however, is accompanied by various difficulties as describedhereinbefore.

In general, in a light-conducting glass structure in which therefractive index is progressively varied in a direction transverse tothat along which incident light is to advance, utilization is made ofthe principle whereby the light gradually bends its advance directiontoward the direction in which the refractive index transverse to thelight advance direction increases.

The radius of curvature, p, of the light advance path may be representedanalytically by the following wellknown equation in terms of therefractive index 7 of the transmitting medium and variation 8n/8H of therefractive index in a direction normal to the light advance path.

Accordingly, this light-conducting glass structure in one embodiment hasa refractive index gradient such that the refractive index at thecentral plane or central axis in the direction of light advance ismaintained at maximum value, and the refractive index progressivelydecreases with increasing distance away from the central plane or axis.At the same time, in the case wherein the length along the central planeof axis of this structure is relatively large, light which has enteredthe glass structure with an incident angle within a specific rangeadvances along the structure in a meandering or undulating path aboutthe central plane or axis.

When an incident light flux 21 is caused to enter one end of alight-conducting glass fibre 22 having a refractive index distributionsuch that this index decreases progressively toward the outer surfacefrom the centre, this light flux advances through the interior of thefibre without being reflected by the fibre outer surface asdiagrammatically illustrated in FIG. 2. Therefore, it is possible toreduce differences in phase velocities of the light flux at the fibreexit end, spreading of the light flux, and also reduce light reflectionlosses.

It is preferable that the refractive index be symmetrical about thecentral part of the fibre in the cross section thereof and moreover,have a distribution such that it decreases progressively from the centreto the outer surface since, then, the differences in phase velocities ofthe light flux at the fibre exit and the light flux spread can befurther reducedv 5 The most preferable refractive index distribution issuch that it can be represented by a quadratic curve of a formcorresponding to the following equation.

N=No(l-ar where:

r is the distance from the fibre centre in a radial di rection;

No is the refractive index of the glass at the central part of thefibre;

N is the glass refractive index at a point at distance r;

and

a is a positive constant.

When an incident light flux having a constant spacial width and aconstant time width is introduced with an incident angle within aspecific range into a glass fibre having a refractive index distributionas defined above, the light flux intimately exits from the fibre exit ina state wherein, its time width is maintained substantially constant andits spacial width is maintained constant without differences in phase.

When this light-conducting glass fibre is curved with a curvature radiusless than a certain limiting value of the radius of curvature, theincident light flux begins to collide against the fibre surface. Thislimiting radius of curvature is determined by the refractive indexdistribution within the fibre. More specifically, this radius ofcurvature decreases as the refractive index gradient, that is, the valueof a in the above Equation (2), is increased.

The refractive index of a glass depends principally on the glasscomposition. Accordingly, a glass structure in which the refractiveindex of the glass interior progressively varies can be produced bycausing it to have a progressively differing glass compositiondistribution. Furthermore, a light-conducting glass fibre in which therefractive index progressively increases from the surface inward can beproduced by causing it to have a glass composition distributiondiffering progressively inward from the surface.

However, glass structures, particularly glass fibres, having glasscomposition distributions such the their refractive indexes varyprogressively have, in general, been accompanied by difficulties such asthe difficulty in selection of the compositions and difficulty in theproduction thereof.

In general, an oxide glass is composed of one or more glassformingoxides (e.g., SiO B and P 0 and other modifying oxides (includingintermediate oxides). The term modifying oxides is herein used todesignate oxides which, independently, do not become glass but arevitrified when they are suitably blended with glass-forming oxides.

As mentioned briefly hereinbefore, we have found that, by causing theconcentrations of at least two of the oxides among the modifying oxideswithin a glass to vary with position, it is possible to produce a glassstructure having a glass composition distribution such that therefractive index progressively varies therein.

More specifically, we have found that a glass composition distributionwherein the proportions of at least two oxides selected from amongoxides which can function as modifying oxides are progressively variedis highly suitable, Examples of such oxides are Li- O, Na O, K 0, Rb O,Cs O, Tl- O, Au O, Ag O, Cu O, MgO. CaO, SrO, BaO, ZnO, CdO, PbO, SnOand 1421203.

In general, the refractive index of a substance has a relationship tothe molecular refraction and molecular volume which are characteristicof that substance, the refractive index increasing with increasingmolecular refraction and with decreasing molecular volume. Furthermore,the molecular refraction is proportional to the polarizability of thesubstance. The molecular refraction of a glass is considered to bedeterminable, as an approximation, by the sum of the individual ionicrefractions.

Accordingly, the qualitative effect of the presence of certain ions onthe refractive index of a glass can be determined by comparing thevalues of electronic polarization per unit volume within the glass ofthe related ions or the values of electronic polarizability/(ion radius)That is, a cation in which this value is large has a tendency tocontribute greatly to the refraction. When the values of this ratio ofelectronic 'polarisabity to (ion radius) within the crystal arecalculated for the monovalent ions respectively of lithium, sodium,potassium, rubidium, cesium, and thallium as representative cationsconstituting modifying oxides, 0.06, 0.48, 0.57, 0.60, 0.74, and 1.57,respectively, are obtained.

However, the values of this ratio within glasses of certain ions do notcoincide, strictly speaking, with those within the crystals of theseions. For example, the refractive indexes of silicate two-compoentglasses each containing 30 mol percent of an oxide of one of the abovementioned monovalent ions ar d three-component glasses containing 20 molpercent of one of the monovalent atoms, 20 mol percent of CaO, and molpercent of SiO are as set forth in Table 1.

When the monovalent ions respectively of lithium, sodium, potassium,rubidium, cesium, and thallium as indicated in Table l are arranged inthe order of magnitude of the ratio of electronic prolarisability to(ion radius) within glass or in the order of degree of contribution toincrease in refractive index, the resulting order is T1, Li, K, Ma, Rb,and Cs, there being almost no difference between K, Na, Rb, and Cs.

Furthermore, the refractive indexes of silicate twocomponent Glasseseach containing 40 mol percent of one oxide selected from PbO, BaO, CdO,SrO, CaO, ZnO, BeO, and MgO and 60 mol percent of SiO are as set forthin Table 2.

Table 2.

Modifying oxide Refractive index SiO- 60 mol7: R0 40 mo17z PbO 1.81

BaO 1.68

CdO 1.64

SrO 1.61

Table 2.-continued Modifying oxide Refractive index SiO 60 mnWr R 40 mol/r CuO 1.59 ZnO 1.58 860 l.54 MgO 1.51

As indicated by Table 2, when these divalent metal ions are arranged inthe order of magnitude of contribution to increase in the glassrefractive index, the resulting order is Pb, Ba, Cd, Sr, Ca, Zn, Be, andMg.

The relationship between the magnitudes of the above mentionedcontribution of the monovalent ions and the divalent ions can bedetermined by comparing the refractive indexes of glasses ofcompositions wherein a monovalent oxide and a divalent oxide have beenmutually substituted in equal mol quantities, for example, by comparingthe refractive index of a glass composed of 60 percent of SiO and 40percent of a divalent metal oxide R0 and the refractive index of a glasscomposed of 60 percent of SiO percent of a divalent metal oxide R0, and20 percent of a monovalent metal oxide R O, all percentages being molpercents.

As a result of an actual instance of such comparison, it was found thatthe above mentioned degrees of contribution of the divalent ions of Pb,Ba, Cd, Sr, Ca, and Zn are greater than those of the monovalent ions ofK, Na, and Li, and that this degree of contribution of the monovalention of TI is greater than that of any divalent metal ion.

The relationships of the magnitudes of this degree of contribution toincrease in refractive index of these cations are also realized in theglass compositions other than those recited in Table l and 2.Accordingly, since the additivity rule of refractive indexes isapproximately realized the relationships of the magnitudes of the degreeof contribution become evident as the magnitudes of the refractive indexof a glass containing these cations even in the case of a compositionother than silicate glass, and even when the component contents differ,as long as glasses wherein the mo] concentrations of the oxides of thecations concerned are mutually equal and, moreover, other constituentsare of the samekind and have mutually equal mol concentrations, arecompared.

As a general tendency, among ions of homologues (for example, among ionsof alkaline earths), those of greater ionic radius and atomic numberhave greater degrees of contribution to increase in the refractiveindex. For example, Ba Sr Ca.

Accordingly, when the refractive index of a first glass containingcertain cations constituting modifying oxides is compared with therefractive index of a second glass in a state wherein one part or all ofthe cations within the first glass are substituted in a manner such thatthe charges of ions are balanced with cations having lower degrees ofcontribution to increase in refractive index within the glass than thecations of the first glass, the latter is less than the former.

Accordingly, the light-conducting glass structure according to thepresent invention contains therein at least two kinds of cationsconstituting modifying oxides and comprises these oxides and glassforming oxides,

Within which glass the refractive index has a distribution such that itvaries progressively with position, more specifically, such that itvaries progressively in a direction transverse to the direction ofadvance of light propagating through the glass, the direction of lightadvance being bent toward the direction in which the refractive indexincreases.

In this glass structure of the invention, in general, the concentrationswithin the glass of the cations vary in the direction in which therefractive index is to vary, and the concentration of certain cationsand the con centration of other cations vary in opposite orientations,the resulting concentration variation thereby creating the variation inrefractive index.

In accordance with the present invention, the above used term at leasttwo kinds of cations designates a combination of metal ions of greatercontribution to increase in the refractive index of the above mentionedglass structure and metal ions of less contribution. The concentrationof the metal ions of greater contribution is higher in a position atwhich the refractive index is to be higher than in a position at whichthe refractive index is to be lower, while the concentration of themetal ions of less contribution is lower in a position at which therefractive index is to be higher than in a position at which therefractive index is to be lower.

Since the cations constituting the modifying oxides can undergomigration Within the glass at a relatively lower temperature than thecations constituting the glass forming oxides, the light-conductingglass structure of the invention, in which the concentration within theglass of the cations constituting modifying oxides is varied, can bereadily caused to have a desired refractive index distribution.Ordinarily, within this light-conducting glass structure, theconcentration of the cations constituting the glass forming oxides doesnot vary remarkably with position or is substantially constant.

The refractive index of the light-conducting glass structure of theinvention varies progressively, as described above, and the refractiveindex distribution within a light-conductin g glass fibre, which is aspecific example of embodiment of the invention, is such that therefractive index increases from the outer surface of the fibre towardthe interior. In this case, while a continuous variation or increase inthe refractive index is preferable, there is no deleterious result evenwith a discontinuous variation or increase of the refractive indexprovided that it is of an order such that the reflection of light at thediscontinuous surface can be neglected.

Examples of cations to constitute the above mentioned modifying oxidesare monovalent cations, for example, alkali metal ions such as Li ions,Na ions, K ions, Rb ions, and Cs ions, Tl ions, Au ions, Ag ions, and Cuions; divalent cations, for example, alkaline earth metal ions such asMg ions, Ca ions, Sr ions, and Ba ions, Zn ions, Cd ions, and Pb ions;trivalent cations, for example, La ions and In ions; and tetravalentcations, for example, Sn ions and Th ions.

Examples of combinations of metal ions for forming at least twomodifying oxides with metal ions of greater contribution and metal ionsof less contribution to increase in the refractive index selected from agroup of specific metal ions as enumerated above are as follows. In eachof these combinations, the first mentioned ion is a metal ion to existwith a higher concentration in a position at which the refractive indexis to be higher than in a position at which the refractive index is tobe lower.

1. Thallium at least one kind of alkali metal ion.

2. Alkaline earth metal ion having a greater atomic number alkalineearth metal ion having a smaller atomic number.

3. Lead ion at least one kind of alkaline earth metal ion.

4. At least one kind of ion selected from the group consisting of leadion and barium ion at least one kind of ion selected from the groupconsisting of K, Na, and Li.

5. At least one kind of ion selected from the group consisting of lead,cadmium, zinc, barium, strontium, and calcium at least one'kind ofalkali metal ion. Among the cations constituting modifying oxides,

monovalent ions can be diffuse through glass at a lower temperature thanother cations. Therefore, by selecting monovalent cations for both ofthe two kinds of cations of increasing and decreasing concentrationswithin the glass, greater convenience is afforded in the production ofglass structures including fibres. Furthermore, a combination of Tl ionsamong monovalent cations and at least one kind of ion of another alkalimetal as, for example, Na, Li K", Rb, and Cs ions facilitates theattainment of large refractive index variations and is, therefore, mostconvenient for the light-conducting structure itself and also for themethod for production thereof as described in detail hereinafter.

The cations constituting the modifying oxides contained within the glassof the invention is not limited in number to two kinds but may be ofthree or more kinds. For example, a glass fibre in which, when threekinds of cations of different values of the aforementioned ratio whichcan constitute modifying oxides are selected and arranged as A ion, Bion, and C ion in the order of magnitude of the aforementioned ratio,the concentration within the glass of the B ion decreases inward fromthe surface, while those of the A ion and C ion both increase inwardfrom the surface can have a refractive index distribution such that theindex progressively increases inward from the surface.

That is, in the case wherein the difference between the aforementionedratios of the C ion and B ion is considerably less than thedifference'between the ratios of the B ion and A ion, or in the casewherein the variation of the concentration of the C ion is less thanthat of the A ion or the B ion, the presence of the A ion and B ioncounteracts the effect of the C ion. As a result, the distribution ofthe refractive index becomes such that the index increases progressivelyinward from the surface. Furthermore, cations constituting othermodifying oxides may be contained without concentration variation withinthe glass.

Thus, a variation in the refractive index is created in the glassstructure of the invention by the variation with position of theconcentrations of at least two kinds of cations constituting modifyingoxides within the glass. This variation in the refractive index is of amagnitude sufficient to provide a light-conducting glass structure whichdoes not have the disadvantageous features observable inlight-conducting structures of known clad type.

More specifically, for example, the difference between the refractiveindexes at the core and surface of a glass structure or fibre inaccordance with the invention having a diameter of less than 5 mm., forexample, from 0.02 to 2 mm., in general, is at least 0.003 for example,from 0.01 to 0.3. Furthermore, in a light-conducting glass structurehaving a relatively large diameter (dimension in the directiontransverse to the direction of light advance) desirably afforded by aclad type light-conducting glass structure, the difference between therefractive indexes at the core and at the surface is of the order offrom 0.03 to 0.3 in the case of a diameter of from 1 to 5 mm.

Glasses suitable for use in the light-conducting glass structure of theinvention are silicate glasses, borate glasses, phosphate glasses, andother oxide glasses. The desirable ranges of the glass compositionalquantities are determined by various factors as, for example, the kindsof modifying oxides, the refractive index distribution to be created,the kinds of oxides to form the glass, the characteristics as a glassmaterial required of the this light-conducting glass structure, and themethod of manufacture.

More specifically, for example, it is not desirable that the quantity ofT1 0 preferably used in the glass of the invention exceed 50 percent byweight. The reason for this is that, when the T1 0 quantity exceeds 50weight percent, an undesirable colouration can readily occur in theglass. On the other hand, moreover, it is preferable that the T1 0quantity be greater than 2 percent by weight at the region where therefractive index is maximum. The reason for this is that, when thisquantity is less than 2 weight percent, a concentration variation forproducing an amply large variation in the refractive index cannot beattained.

Accordingly, one example of the glass structure of the inventioncontains from 2 to 40 percent by weight of T1 0 at the region where therefractive index is maximum, from 35 to percent by weight of SiO andfrom 0 to 40 percent by weight of one or more alkali metal oxides(preferably Na O and/or K 0).

The principal specifications, such as the glass composition (expressedin mole percents and, when differing at the centre and surface, soindicated), glass structure diameter, refractive indexes at the centreand surface, and the value of a in the previously set forth Equation (2)in the vicinity of the centre of examples of the glass structureaccording to the invention are set forth in Table 3.

SiO

Table 3.-continued Glass 1 2 3 structure: Part: Ccntre Sur- Centre Sur-Centre Surfacc face face B 0,- 70 A5 0 0.1 Diameter, mm. 0.3 0.5 0.3Refractive index centre: 1.60 1.58 1.81 surface: 1.57 1.55 1.77

a near centrc cm 62 30 30 In the light-conducting glass structure of theinvention, the path of advance of light introduced thereinto is bentwithout the occurence of reflection within the glass structure of thelight flux. That is, an incident light introduced into the glassstructure advances therethrough as it curves toward the side of higherrefractive index. Therefore, it is possible to bend the direction ofadvance of light through the use of the light-conducting glass structureof the invention.

Furthermore, a light-conducting glass structure having a refractiveindex distribution which is symmetrical about the central axis of theglass in the light transmission direction and is of a character suchthat the refractive index decreases or increases approximatelyproportionally to the square of the distance from the axis as thedistance increases, particularly such a glass structure which has beencut to a specific length, can be caused to operate as a convex orconcave lens such as to cause converging or diverging of a light flux.

In a light-conducting glass fibre of the invention, since an incidentlight flux introduced thereinto advances therewithin and therealongwithout being reflected, differences or discrepancies in the phases ofthe rays in the light flux exiting from the fibre do not occur, and,moreover, spreading of the light flux width is prevented. Therefore, byutilizing a light-conducting glass fibre according to the invention inphotocommunication, it becomes possible to transmit with high efficiencylight signals which vary at high speeds.

By providing a light-conducting glass fibre in one part of a lighttransmission path in photocommunication, and by utilizing the flexibleand bendable property possessed by the fibre, it is possible to adjustas desired the position in space and projection direction of lightprojected out of the fibre exit end.

Particularly when an incident light flux of constant width is introducedby a so-called mode-matching or mode-conformity method into a glassstructure having a refractive index distribution representable byEquation (2) set forth hereinbefore, the light flux can be conductedthrough the structure to be projected from the structure and exittherefrom with the same constant width and without differences in phasevelocities even when the structure is bent with a radius of curvaturegreater than a certain limiting value of radius of curvature. The lightflux can also follow a curve within certain limits without anaccompanying diffraction loss.

These features of performance can be readily understood through analogyfrom the theory of gas lenses described in the aforementionedreferences. Therefore, glass structures having such a refractive indexdistribu tion are highly effective for use as transmission lines forlaser communication.

Furthermore, in glass structures of plate shape or tubular shape, it ispossible to establish a refractive index distribution such that theinternal refractive index decreases in approximate proportion to thesquare of the distance from approximately the central plane. Whenincident light is introduced along the central plane of this glassstructure, the light advances as it meanders through an undulating pathon both sides of the central plane. Such a glass structure can be usedas a light-conducting glass structure in applications such as lenticularlenses.

It has always been necessary for a light-conducting glass fibre of knownclad type to have a glass cladding layer for reflection. In contrast, acladding layer for reflection is not always necessary for thelight-conducting glass fibre of the present invention. Accordingly, theeffective area for light-conduction is relatively large, and themanufacture of the fibre is facilitated.

The light-conducting glass fibre of the invention can, in accordancewith necessity, also be clad over its surface with a substance which hasa refractive index different from that of the glass fibre, such as alightabsorbing substance, or a light-reflecting substance. It is alsopossible to assemble a plurality of these glass fibres as a fibre bundleor as a densely packed bundle structure. Furthermore, thelight-conducting glass fibre of the invention can be applied tocommunication by laser and other kinds of light, to various kinds ofimage transmission, and to other light-transmission apparatuses andsystems.

It will be readily evident that the articles referred to herein aslight-conducting structures can have shapes and dimensions within verybroad ranges. The term fibre is herein used to designate structureshaving small cross sections in comparison with their lengthsirrespective of the shapes of their cross sections. The cross section ofsuch a fibre can be circular, polygonal, or some other shape.

A light-conducting glass structure of the above described character inaccordance with the invention can be produced by a method which includesa process of ion substitution by thermal diffusion to cause a variationwith position of the concentrations of at least two kinds of cationsconstituting modifying oxides within the glass.

Accordingly, the method according to the invention for producing alight-conducting glass structure having a refractive index distributionand light-path bending property as described above comprises, ingeneral, a greater substitution of the cation constituting a modifyingoxide of the glass at parts closer to the glass surface with anothercation capable of constituting a modifying oxide. This ion substitutioncan be accomplished by causing the glass containing the above mentionedcat- 13 ion to contact the source of the other cation as the processtemperature is maintained at a value such that the above mentioned twokinds of ions can migrate within the glass. This substitution producesthe variation of refractive index within the glass.

The above mentioned source of the other cation can be of various kindsand forms. For example, it can be a bath of a salt such as a nitrate orsulphate, an oxide, or an elementary simple substance containing thiscation. This bath can be in the state of a solid, liquid, or a vapour.Another example of the cation course is a glass containing this ion in aform such as an oxide, for example.

The above mentioned temperature at which the contact with the ion sourceis carried out is, ordinarily, higher than 100C and, moreover lower thanthe flow point of the glass. When this temperature is below 100C, themigratory speeds of the ions are low, whereby the practicality of theprocess is low. On the other hand, when this temperature is higher thanthe glass flow point, it is difficult to avoid undesirable deformationof the glass.

More specifically, this contacting temperature is preferably higher thanthe glass strain point and, moreover, lower than the glass softeningpoint. Most preferably, this temperature is slightly higher than theglass deformation temperature. For example, this process temperature ismost preferably in the range of from 450 to 500C for a glass containing16 percent of T1 0, 24 percent of PbO, 12 percent of Na O, and 48percent of SiO (all percentages by weight) since the strain point ofthis glass is approximately 350C, and the softening point thereof isapproximately 565C.

Accordingly, in one specific example of method for producing alight-conducting glass structure of the invention, the glass structureis placed in contact with a salt, and the glass and salt are heated andmaintained at a temperature at which the salt and the cation in theglass can diffuse within the glass. As the cation within the saltdiffuses into the interior of the glass through the contact surfaces ofthe salt and glass, a portion of the cation hitherto present in theglass diffuses and moves out of the glass. As a result, the cation inthe glass near the contact surface is substituted by the cationpreviously contained in the salt.

The concentration within the glass of the cation which has diffused fromthe salt into the glass interior is higher at points nearer the contactsurface and decreases with the distance from the contact surface.Inversely, the concentration within the glass of the cation previouslyexisting within the glass is lower at points nearer the contact surfaceand increases with the distance from the contact surface.

When ratio of the electronic polarizability of the cation contained inthe salt and (ion radius) (more precisely, the degree of contribution toincrease in the refractive index) is lower than that of the cation previously contained within the glass, the refractive index of the glassafter ion substitution is lower at parts nearer the contact surface andassumes a distribution wherein the index increases progressively withdistance from the contact surface to approach the original glassrefractive index. lnversely, when the above mentioned ratio or degree ofcontribution of the cation contained in the salt is higher than that ofthe cation contained in the glass, the refractive index of the glassafter ion substitution is higher at parts nearer the contact surface andassumes a distribution wherein the index decreases 14 progressively withdistance from the contact surface to approach the original glassrefractive index.

By selecting a salt and a glass to produce the former distribution andcausing the salt to contact the surface of the glass in the form of afibre with a solid interior thereby to cause ion substitution, alight-conducting glass fibre of the invention in which the refractiveindex increases progressively inward from the surface toward theinterior can be produced. By selecting a salt and a glass for the lattercase, causing the salt to contact the inner wall surface of the glass inthe form of a fibre with a hollow interior to cause ion substitution,and then causing the inner wall surface to contract and collapse byheating and deforming the glass, a light-conducting glass fibre of theinvention having a shape with a solid interior can be produced.

The refractive index within a glass fibre of the invention is determinedprincipally by the following conditions. In a glass of fibre form whichhas been subjected to the ion-substitution process, the refractive indexis determined by conditions such as the composition, dimensions, andshape of the fibrous glass, the composition of the salt, and thetemperature and time duration of the ion diffusion process. Since thequantity of diffusion of the ions within the glass is determined by thedistance from the contact surface of the glass with the salt,particularly in the case wherein a glass structure of circular or aconcentrically circular cross section is used as the glass of fibrousform, the refractive index of the glass interior after ion substitutionassumes a symmetrical distribution wherein the index is determined bythe distance from the control part in thecross section of the fibre,whereby a desirable result is obtained.

Furthermore, by carefully selecting the process conditions, it ispossible to cause the refractive index to approach an ideal distributionrepresentable by a quadratic equation. In addition, it is possible, bysubjecting the processed glass in fibrous form to a deformationfabrication process, as necessary, by a treatment such as heating andstretching or surface treatment, to render the glass into alight-conducting glass fibre having desired dimensions and refractiveindex distribution.

We have found, for example, that by subjecting a glass rod of a diameterof from 0.5 to 2 mm. to ion substitution process and then to a heatingand stretching process, a light-conducting glass fibre of across-sectional distribution of refractive index which is uniform in thelongitudinal direction, of long length, and of a diameter of from 10 to200 microns can be easily produced.

For the salt to be used as an ion source, any of the specificallyenumerated salts containing a cation can be used. It is preferable touse a salt containing the cation existing within the glass and a cationconstituting any of the combinations 1) through 5), inclusive,enumerated hereinbefore.

A salt suitable for use in accordance with the invention is a saltcontaining one or more kinds of these ions. Furthermore, it is alsopossible to use a mixture of this salt and a salt containing a kind ofion other than that of the above mentioned ion. It is necessary thatthis salt have a suitable melting point since the cation within the saltmust diffuse into the glass interior at the temperature of thesalt-to-glass contacting process. Ordinarily, salts such as nitrates andsulphates containing the above mentioned cation are suitable.

In some cases wherein, of the above mentioned cations, Au ions, Ag ions,or Cu ions are caused to be contained within the glass or salt, theionic valency of these ions vary during the process, undesirablecolouration occurring in some instances. Accordingly, in order toprevent this variation, it is necessary in such cases to maintainconditions such as the glass composition and environmental atmosphere ina manner such that an oxidation or reduction reaction does not readilyoccur.

Tl ions have a relatively higher degree of contribution to increase inrefractive index than ions of alkali metals such as rubidium andpotassium. Moreover, the radius of the Tl ion or its ion volume does notdiffer greatly from the ion radius or ion volume of metals such asrubidium and potassium. For this reason, when a glass containingthallium and a source containing an alkali metal ion such as potassiumor rubidium are placed in contact with each other at a high temperature,a light-conducting glass structure having a large refractive indexgradient wherein the index increases progressively inward from thesurface and having relatively low residual stress is produced.

Since a potassium source is more readily available than a rubidiumsource, a glass containing thallium is ordinarily caused to contact apotassium source. While sodium ion is considerably smaller in ion radiusthan a potassium ion, the aforementioned contributions of these twometals are substantially equal. Then, when a glass containing Tl and Nais placed in contact with a potassium source, the Tl and l\la arerespectively substituted by K*, but the substitution of Na and K in thiscase imparts almost no effect with respect to the refractive indexdistribution. On the other hand, this substitution has the effect oftending to nullify the development of tensile stress due to ionsubstitution of Tl and K and thereby tends to produce highly desirableresults.

While residual stress develops within a glass in which ion substitutionis being effected, this residual stress, when of large magnitude, tendsto give rise to deleterious occurrences such as cracks in the glass.Accordingly, it is desirable that this residual stress be held below acertain value. The value of this residual stress can be lowered byexpedients such as maintaining the ion diffusion temperature at a highvalue thereby to relieve and lessen stain, subjecting the glassstructure to a heat treatment after ion substitution, and appropriatelyselecting the compositions of the glass and the salt.

When the process temperature is set at a value higher than the annealingpoint, the possibility of deformation of the glass being processedincreases, but the stresses within the glass due to difference in thevolumes of the ions undergoing substitution can be relieved and removedthrough lengthening the time of treatment sufficiently. We have found,furthermore, that differences in coefficients of thermal expansion ofglass compositions which vary with position due to ion substitution(which differences also tend to produce residual stress) are ofmagnitudes which are not deleterious in the practice of this invention.

The development of this residual stress can be prevented by thefollowing specific example of embodiment of the invention.

A glass structure containing a first cation (e.g., thallium ion) iscaused to contact an ion source containing a small quantity of the firstcation and a second cation (e.g., an alkali metal ion) of a ratio ofelectronic polarizability to (ion radius) (more correctly, degree ofcontribution to increase in refractive index) which differs from that ofthe first cation thereby to cause 16 contact between the first cationwithin the glass and the second cation within the ion source in regionsnear the contact surfaces and thereby to cause the refractive index tovary progressively inward from the glass surface.

In a process as described above, since a small quantity of the firstcation in addition to the second cation is contained in the salt or someother ion source placed in contact with the glass containing the firstcation, the presence of the first ion within the ion source has theeffect of obstructing the diffusion and migration of the first ionpreviously in the glass into the ion source. Then the substitution ofthe first and second ions within the glass near the surface thereofdecreases relative to that occurring when the first ion is not containedin the ion source. As a result, the magnitude of the internal stress islowered. Therefore, even when the contacting process temperature is heldat a high value, there is no risk of development of fine cracks.

By thus maintaining the contacting temperature at a high value, thediffusing speeds of the first and second ions within the glass areincreased, whereby it is possible to produce with a short contactingtime a light-conducting glass structure of a desired refractive indexdistribution.

Even an extremely small quantity of the first ion contained in the ionsource is highly effective, ordinarily being of the order of from 0.1percent to a number of percent by weight.

In an actual instance of practice, for example, two pieces of glass rodsof approximately 0.5 mm. diameter having a glass composition in weightpercents of T1 0 5, PbQ 20, Na O 15, and SiO 60 were immersed in amolten bath of KNO at 450C for 24 and 72 hours respectively. Thereafter,the internal refractive index distributions of the two glass rods weremeasured, whereupon values as indicated by the intermittent line andcontinuous line in FIG. 7 were obtained.

A glass rod of the same composition and dimensions as set forth abovewas immersed in the same bath at 480C, whereupon fine cracks developedin the surface of the glass rod. In addition, a glass rod of the samecomposition and dimensions as set forth above was immersed for 24 hoursin molten salt mixture of 99.5 percent by weight of KNO and 0.5 percentby weight of TiNO at 480C, whereupon the glass rod was found to have aninternal refractive index distribution coinciding with that indicated bythe continuous curve in FIG. 7 without any fine cracks on its surface.Thus, by using a salt mixture of TiNo and KNO a glass rod having arefractive index distribution wherein the index decreases approximatelyin proportion to the square of the distance from the centre can beproduced by a steeping process of relative short time duration.

By the procedure of the above describe example, furthermore, theintroduction of a large quantity of the first ion, particularly an ionconstituting a modifying oxide, into the glass structure becomespossible, whereby it has become possible to improve greatly the qualityof the glass.

That is, more specifically, in order to produce a glass without bubblesand striae, it is preferable to cause the glass to contain a quantity ofmodifying oxides sufficient for carrying out the work of uniformisingthe quality of the glass, but an increase in the modifying oxidequantity tends to promote development of fine cracks in the glass,wherefore this increase has hitherto been limited. By the elimination asdescribed above of 17 the risk of this fibre formation, it has becomepossible to adjust the quantity of modifying oxides in the glass to aquantity amply large for producing a glass of excellent quality.

The refractive index gradient of the glass structure, particularly thepreferable internal refractive index distribution in the case of alight-conducting glass structure of rod shape or fibre shape can berepresented by Equation (2), in which the value of the constant a is animportant factor determining the optical characteristics of the glassstructure. By this specific example, a glass structure having a desiredvalue of a can be obtained merely by appropriately selecting thequantity of the aforementioned first ion admixed within the ion sourceto be placed in contact with the glass. The effect of adjustment of thequantity of this first ion contained in the ion source on the refractiveindex distribution is indicated by the following example.

Glass rod samples of approximately 0.6 mm. diameter having a compositionby weight of 20 percent of Tl O. 10 percent of PbO, 14 percent of NaOand 56 percent of SiO were respectively immersed for 22 hours in fourkinds of single-salt and salt-mixture baths, as follows: (1) 100 percentof KNO (2) 99.5 percent of KNO aand 0.5 percent ofTiNO N (3) 99.3percent of KNQ, and 0.7 percent of TiNO and (4) 99.1 percent of KNO and0.9 percent of TiNO (said percent referring to weight percent). Theresulting refractive index distributions within the glass rod sampleswere as indicated in FIG. 8, in which curves 81, 82, 83 and 84correspond to the results of samples immersed in salt baths (1), (2),(3), and (4), respectively.

FIG. 8 indicates that the admixture of only minute quantities of TiNOcauses a great change in the resulting refractive index distribution.When, of these distributions, those of the parts near the central partsof the glass rods are approximated by Equation (2), the values of a inthe vicinity of the centres corresponding to salts (1) through (4) are57, 37, 26, and I1 cmf respectively. The value of No in this case isl.563 and is commonly equal.

In this case the curve of the distribution resulting from the processwith salt (2) is the closest to a quadratic curve. The ion concentrationdistributions of TI", K", and Na within the glass after processing withsalt baths (2) and (4) are respectively indicated in FIGS. 9 and 10, ineach of which the concentration distributions of Tl K and Na areindicated by a continuous line, an intermittent line, and a dot-and-dashline, respectively: The concentration taken as the ordinate in each ofFIGS. 9 and 10 is to an arbitrary scale.

From FIGS. 9 and 10, it can be observed that an increase in theconcentration of TiNO among the salts from 0.5 percent to 0.9 percentresults in a lowering of the rate of the proportion of decrease in theion concentration of Tl from the glass interior outward toward thesurface.

Except for the feature of admixing into the ion source to be placed incontact with the glass the same ion as the ion within the glass to besubstituted, this specific example does not differ essentially from thefirst described specific example. Each of the first and second ions isnot necessarily limited to a single kind but may contain a plurality ofkinds of ions. Furthermore, the glass to be processed may contain, inaddition to the first ion, other ions to be substituted by ions withinthe salt. In the case of this example, also, it is preferable 18 thatthe glass structure contain Tl and, moreover, that the ion sourcecontain alkali metal ions.

In the case wherein a glass fibre containing ions of high degree ofcontribution to increase in the refractive index as, for example,thallium ions, is placed in contact with a source of ions on low degreeof contribution as, for example, alkali metal ions, thereby to causesubstitution of the ions, the concentration distribution of each kind ofion is derivable as a solution of the diffusion equation, and, as aresult of the process during the time until the ions of the source reachat least the centre of the fibre by diffusion, a refractive indexdistribution conforming to an ideal quadratic curve is produced near atleast the centre of the fibre structure.

However, in spite of the establishment of a refractive indexdistribution of this nature near the centre, the refractive index of thefibre interior nearer the surface tends to deviate easily and moregreatly from the above described distribution. Since this surface layeraccompanied by this deviation impairs the light-conductingcharacteristics of the fibre structure, it is preferable to remove thislayer. Ordinarily, the excellent light-conducting characteristics can bepreserved by removing a surface layer of a thickness which isapproximately 20 percent of the radius of the fibre structure.

While this surface layer removal is carried out along the periphery ofthe fibre as viewed in cross section, it is not necessarily carried outover the entire length of the fibre in the longitudinal directionthereof, the necessary length of removal being at least WV 2 a. Whilethis surface layer removal is most preferably accomplished by etchingthe fibre with an aqueous solution of hydrofluoric acid, the fibresurface alternatively may be removed by grinding.

Another example of the aforementioned source of another cation is aglass containing this ion. This ion source is caused to contact a baseglass structure to be subjected to ion exchange by covering the baseglass structure with this ion source glass, causing the two glasses tofuse together, and maintaining the resulting fused structure at atemperature at which the two kinds of ions can migrate.

In one specific example of this case, the base glass structure is thinand long and has at least one circular boundary as viewed in its crosssection, and the region inside of this boundary contains Tl ions, whilethe region outside thereof contains at least one kind of ion from amongLi Na, K*, Rb, and Cs ions. This glass structure is held at an elevatedtemperature to cause ion exchange through the above mentioned boundary.As a result, the glass inside of the boundary is caused to have arefractive index distribution wherein the index decreases progressivelyfrom the centre outward toward the periphery.

The manner in which a refractive index distribution as described abovecan be realized within a glass structure will now be, described.

When two kinds of glasses with different compositions are fused togetherwith a smooth boundary interface therebetween and are maintained at ahigh temperature for a long time, an exchange of ions due toconcentration diffusion occurs through the interface. As a result, asmooth ion concentration distribution is produced in the directionperpendicular to the interface. The temperature at which the two glassesare thus treated is preferably of an order such that there is no severedeformation due to softening of the glasses,

19 being ordinarily below the softening temperature of the glasses.

The kinds of ions which undergo such ion exchange with particularfacility, however, are limited to monovalent metal ions as is apparentfrom a comparison of the degree of ion transmission of the glasses.Specific examples of such ions are Li*, Na, KF, Rb, Cs and T1 We havefound that in the case wherein, of the two. kinds of glasses fusedtogether, a first glass contains Tl while the second glass contains atleast one kind of ion from among Li*, Na", K Rb*, and Cs, a refractiveindex gradient which is sample for light conduction can be obtained. 7 1

The ion concentration distribution established in the directionperpendicular to a smooth boundary interface by ion exchange through theboundary interface assumes the character of a smooth curve, but since adiffusion phenomenon is utilized therefor, this distribu' tion may beunderstood as being in accordance with a solution of the so-calleddiffusion equation.

Then, a combination of the first glass disposed inside of a circle (asviewed in cross section) and the second glass disposed outside of thecircle, which first glass contains an ion of large value of theaforementioned ratio of electronic polarizability to (ion radius) (morecorrectly, degree of contribution to increase in the re fractive index),i.e. T/t and which second glass contains an amply small kind of ion,e.g., Na of the same ratio may be considered. When such a glasscombination is maintained for a long time at a high temperature at whichthe deformation of the glasses is not severe, the TI* ions escapeoutward into the region outside of the circle, and the Na ions migrateinto-the region inside of the circle.

As a result, the T1 and Na ions in the region of the first glass assumeconcentration distributions wherein the Tl ions are most dense at thecentre and become progressively less dense with distance from thecentre, while the Na ions are least dense at the centre and becomeprogressively dense with distance from the centre.

In an actual instance of preparation of a glass rod comprising a firstglass composed of 30.7 percent of SiO 52.6 percent of PbO, and 16.7percent of T1 and a second glass composed of 37.2 percent of SiO 50.0percent of PbO, and 12.8 percent of Na O (all percentages being byweight), of heat treating this glass rod at a high temperature, and ofmeasurement of the concentration distributions of the Tl and Na ionsover the cross section by means of an electron microprobe X-rayanalyzer, results as indicated in FIG. 11 were obtained.

In FIG. 11(a), the intermittent line indicates the boundary interfacebetween the two glasses, and the distributions of the concentrations ofTI ions which were contained in the first glass on the left side of thisinterface and of Na ions which were contained in the second glass on theright side after the glasses had been treated at 430C for 8 hours arerespectively indicated by continuous lines and dot-and-dash lines. Thecorresponding concentration distributions in the case wherein, at thesame temperature, the treatment time was increased to 48 hours areindicated in FIG. 11(1)). As indicated by these graphicalrepresentations, the distributions of the Tl and Na ions within thefirst glass progressively become smooth.

The resulting refractive index distributions of the glass subjected tothe above described heat treatments are indicated in FIGS. 12(0) and12(1)), which correspond respectively to FIGS. 11(a) and I1(u). Asindicated in these FIGS., when heat treatment is carried out thoroughly,a thin glass structure having in the neighbourhood of its centre axis aregion wherein the refractive index decreases with a form approaching aquadratic distribution from the centre axis toward the periphery can beproduced. This region,as stated hereinbefore, is capable of conductingwith high fidelity a light beam containing a large quantity ofinformation signals. That is, it is possible to provide a glass havingthe capability of transmitting pictures and images.

A glass structure of the above described character according to theinvention can be produced by the use of apparatuses of the followingdescription.

To produce a glass structure having a boundary interface and a firstglass within the hollow cylindrical interior of a second glass, a glassstructure 131 of rod shape and of a first glass composition isencompassed by a tubular glass structure 132 of a second glasscomposition, and the two glass structures 131 and 132 are fusedintimately together by means of a heating device such as a furnace 133as indicated in FIG. 13, the glass structure thus fused together beingstretched by rolls 134 thereby to produce the desired glass structure.

Another technique in producing the double glass structure comprisesplacing the glass 141 of the first composition in the inner pot 142 ofdouble concentric pots made of platinum and having funnel-like shapeswith constricted concentric nozzle openings at their bottoms as shown inFIG. 14, placing the glass 143 of the second composition in the outerpot 144, resistance heating the platinum pots to melt the two glasses,and drawing the glasses downward through the nozzle openings.

In another embodiment of the invention as shown in FIG. 15, a glassstructure is formed with two or more independent bodies of a first glass15 la through 151g disposed within a second glass 152. A glass structureof this character can be produced, for example, by a method as indicatedin FIG. 16. Double concentric pots consisting of an inner pot 161 and anouter pot 163 made of platinum and having a funnel-like shape withconstricted nozzle openings at their bottoms are used. The inner pot 161has two or more nozzle tips 162 at its bottom, while the outer pot 163has a single annular nozzle tip. A first glass 164 is placed in theinner pot 161, and a second glass 165 is placed in the outer pot 163.The platinum pots are then resistance heated to melt the glasses, whichare drawn downward through the nozzles similarly as in the exampleillustrated in FIG. 14.

Furthermore, a plurality of glass structures each fabricated by themethod indicated in FIG. 13 or FIG. 14 and having a single core can befused together and drawn by holding one end of a bundle of the glassstructures 171 by means of a chuck 172 and pulling the remainder of thebundle by means of rolls 173a and 173b as indicated in FIG. 17. As aresult, a glass structure having a cross section wherein rod-like bodiesof the first glass are disposed within a plurality of respective ringsis obtained.

In a glass structure produced by any of the above described methods andhaving glasses mutually fused at a fusion interface, a very slightdegree of ion exchange is obtained through the fusion interface, butthis is not sufficient for producing the desired refractive indexgradient, ,and a heat treatment is ordinarily necessary.

21 Preferably, the glass structure is maintained for a long time at atemperature below the softening temperatures of the glasses.

The glass structure of rod shape thus obtained can be rendered into aglass of fibrous form by heating the glass structure after ample heattreatment thereof to a temperature above the glass softening point andstretching the structure thus heated.

It is important that the two kinds of glass to be used as describedabove be so selected that their coefficients of thermal expansion aresufficiently close to each other. However, since the glass structure isin the form of a thin rod or a fibre, it is possible to suppress theoccurrence of double refraction even when the two coefficients are notstrictly coincident. Furthermore, since the heat treatment time is long,it is necessary to select glasses of ample small devitrificationtendency.

Very fine glass fibres produced in the above described manner can beassembled to form an optical fibre bundle. We have found that thequantity of light transmitted through such a fibre bundle is muchgreater than that transmitted through conventional optical fibres ofclad type. The reason for this high performance is that loss due tototal reflection does not arise in the optical fibre of the invention.

Techniques such as using a tinted glass for the second glass thereby toprevent infiltration of light from the outside and effecting lightinsulation mutually between the members of the first glass are effectivefor maintaining contrast of images and signals.

When, in a composite glass structure comprising a core glass structureand a covering glass which, as an ion source, has caused a desiredvariation in the refractive index within the core glass, the refractiveindex variation created also in the covering glass is considered, andthe refractive index variation across the interface between the twoglasses is a smooth variation, this composite glass in entirety becomesa light-conducting structure according to the invention.

More specifically, in one specific example of embodiment of theinvention, there is provided a method for producing a light-conductingglass structure in which the refractive index is caused to varyprogressively in the transverse direction relative to the light advancedirection thereby to cause the light advance path within the structureis caused to band, the method comprising the steps of combining inlaminar disposition glass members of at least two different refractiveindexes and holding the glass members thus combined at a temperature atwhich diffusion through the interface therebetween of ions constitutingmodifying oxides within the glasses on both sides of the interfaceoccur.

In a glass structure to be thus produced, oxides including ions whichhave not undergone diffusion, within the two glasses are so selectedthat in the case wherein they mutually have substantially equal molconcentrations and are formed into independent glasses, these glasseshave substantially equal refractive indexes.

In order to indicate still more fully the nature and utility of theinvention, the following specific examples of practice constitutingpreferred embodiments of the invention are set forth, it beingunderstood that these examples are presented as illustrative only andthat they are not intended to limit the scope of the invention.Throughout these examples, all percentages of quantity are by weightunless otherwise specified.

EXAMPLE 1 A glass fibre having a 0.2 mm. diameter and composed of 20percent of Tl O, 12 percent of Na O, 20 percent of PbO, and 48 percentof SiO was steeped for 4 hours in a potassium nitrate bath maintained at400C. Thereafter, the glass fibre was removed from the bath, cooled,rinsed with water, and dried.

The refractive index of the surface of the glass fibre thus treated wasfound to be 1.578, which was 0.026 lower than that before the glassfibre was treated. The refractive index distribution of this glass fibreafter treatment is indicated in FIG. 3.

Next, the concentration distributions respectively of thallium ions,sodium ions, and potassium ions with the glass fibre thus treated weremeasured by an electron microproble X-ray analysis method, whereuponresults as indicated in FIG. 4 were obtained. The concentrations shownin FIG. 4 are set forth in accordance with an arbitrary scale.

From these results, it is apparent that the thallium ions, while beingof substantially constant concentration in parts deeper thanapproximately 40 microns from the surface, decrease in concentrationfrom a depth of 40 microns to the surface. The sodium ions assume asubstantially constant concentration in parts deeper than approximatelymicrons and decrease in concentration from 80 microns to the surface. Incontrast, the concentration of the potassium ions increases toward theglass surface and decreases inward from the surface, becoming Zero (nil)at a depth of approximately 80 microns.

These results indicate and verify that the thallim and sodium within theglass were exchanged with the potassium within the bath. Furthermore, itmay be considered that the refractive index distribution as shown inFIG. 3 was produced principally by the exchange of thallium andpotassium ions.

A piece of approximately 10cm. length of this glass fibre was bent toassume a radius of curvature of 1 cm., and an incident light flux of awidth of approximately 0.02 mm. was introduced into the central part ofone end of the glass fibre piece, whereupon the light flux advancedthrough the glass fibre with an undulating path without being reflectedat the fibre surface until it reached the other end of the glass fibre.The width of the light flux reaching this other end was found to besubstantially equal to that of the incident light flux.

EXAMPLE 2 A glass fibre of the same composition and dimension as thatspecified in Example 1 was steeped for 19 hours in a potassium nitratebath maintained at a temperature of 430C. Thereafter the glass fibre wasremoved from the bath, cooled, rinsed with water, and dried.

The refractive index distribution of the glass fibre thus treated wasmeasured and found to be as indicated in FIG. 5, which shows therefractive index as increasing progressively up to a depth from theglass surface of approximately 50 microns.

EXAMPLE 3 Samples of glass fibres of 0.2 mm. diameter respectively of 8different glass compositions were prepared and steeped in a bath ofpotassium nitrate. Thereafter, each fibre was removed from the bath,cooled, rinsed with water, and dried.

EXAMPLE 5 A glass rod composed of 5 percent of T1 0, percent of Na O,percent of PbO, and 60 percent of SiO and having a diameter of 1 mm. wassteeped for 72 hours in a potassium nitrate bath maintained at 430C. Therod was thereafter removed from the bath, cooled,

Table 5. rinsed with water, and dried. The glass rod thus treated Table4 was then heated to approximately 650C and drawn in 10 the longitudinaldirection into a glass fibre of O. 14 mm. i Kfi diameter.

GLASS COMPOSITION by weight) 5102 B203 A120a o Nazo T120 The refract1veindex of the resultlng glass fibre was 1 68 15 found to be 1.545 at thesurface of the fibre and 1.564 2 60 15 at the central part thereof. Therefractive index distrii 2; 15 bution of this glass fibre is indicatedby the continuous 5 63 I 18 10 line in FIG. 6, which line is close tothe quadratic curve 6 56 8 16 20 (intermittent line) of the ideal case.7 49 7 14 8 48 5 15 12 20 EXAMPLE 6 20 A glass fibre composed of 16percent of T1 0, 12

Table 5.

SURFACE REFRACTIVE REFRACTIVE GLASS PROCESS PROCESS INDEX INDEX SAMPLETEMP. TIME BEFORE AFTER DIFFERENCE No. (C) 111. PROCESS PROCESS (A) (B)From the results indicated in Tables 4 and 5, it is apparent that therefractive index difference increases with increase in the processtemperature. It was observed, moreover, that the refractive index of theglass fibre after the treatment at the central part was equal to that ofthe surface of the fibre before treatment and that the refractive indexprogressively decreased toward the surface.

EXAMPLE 4 A glass fibre having a diameter of 0.6 mm; and composed of 16percent of T1 0, 12 percent of Na O, 24 percent of PbO, 2 percent ofCaO, and 46 percent of SiO was steeped for 170 hours in a salt mixturemaintained at 500C and containing 50 mol percent each of K SO. and ZnSOThe internal refractive index distribution of the glass fibre thustreated was measured and found to be such that in the vicinity of thefibre centre, that is, within a circle of 0.25 mm. radius from thecenter as viewed in cross section, the value of the refractive index nwas representable in terms of the distance r (cm.) from the centre and a30 cm substantially in accordance with the following equation.

11 1.60 (l ur In addition, it was verified that this refractive indexdistribution was due principally to the substitution of the thalliumions within the glass and the potassium ions within the salt mixture.

percent of Na O, 24 percent of PbO, and 48 percent of SiO and having adiameter of 0.3 mm. was steeped for 36 hours in a bath of sodium nitratemaintained at 450C.

The internal refractive index of the glass fibre thus treated was foundto exhibit a distribution substantially confroming to the followingequation in the vicinity of the fibre centre, that is, within a circleof 0.1 mm. ra-

dius from the centre 11 1.60 (I ar where: n is the refractive index at adistance r from the centre, and

a cm"? EXAMPLE 7 25 This glass rod was cut to a specific length, andboth cut ends thereof were ground to surfaces perpendicular to thelongitudinal direction of the glass rod, whereupon it was confirmed thatthis rod had the functional effect of a concave lens having an opticalaxic coincident with the centre axis of the rod.

EXAMPLE 8 A glass fibre composed of 62 percent of PbO, 12 percent ofBaO, and 26 percent of SiO; and having a diameter of 0.3mm. was steepedfor approximately 200 hours in a ZnCl bath maintained at 400C.

The internal refractive index of the glass fibre thus treated had adistribution such that in the vicinity of the fibre centre, that is,within a circle of a 0.1 mm. radius from the centre as viewed in crosssection, the refractive index n at a distance r (cm.) from the centrewas substantially in accordance with the following equation.

n 1.81 (l ar' where:

u 30 cm EXAMPLE 9 A glass fibre composed of 27 percent of T1 0, 10percent of U 0, and 63 percent of B and having a diameter of 0.5 mm. wassteeped for 48 hours in a potassium nitrate bath maintained at 480C.

The refractive index of the glass fibre thus treated had a distributionwherein, in the vicinity of the fibre centre, that is, within a circleof a 0.5 mm. radius from the centre as viewed in cross section, theindex n at a distance r (em) from the centre substantially satisfied thefollowing equation.

n 1.58 (1 ar where:

a 30 cm EXAMPLE A hollow glass rod composed of 10 percent of K 0, 10percent of Na O, percent of CaO, and 65 percent of SiO and having anouter diameter of 2 mm. and an inner diameter of 0.5 mm., one end of thehollow interior of the rod being closed, was filled with molten thalliumnitrate and maintained in this state at 400 for hours. Thereafter, thethallium nitrate was drained out, and the glass rod was rinsed withwater.

Next, this glass rod was heated to and maintained at 650C and, as thehollow interior of the rod in this state was evacuated, was drawn in thelongitudinal direction thereof. As a result, the hollow interior of therod became progressively smaller until it collapsed, and, finally, aglass fibre of 0.2 mm. diameter and solid interior was obtained.

The refractive indexes of this glass fibre at the surface and centralpart thereof were found to be 1.535 and 1.554, respectively, therefractive index decreasing progressively from the central part towardthe surface.

EXAM PLE l 1 A glass rod composed of 20 percent of Li O and 80 percentof SiO and having the same dimensions as the glass rod in Example 10 wassubjected to the same treatment as set forth in Example 10, whereupon aglass fibre was obtained.

The refractive indexes at the surface and central part of this glassfibre were found to be 1.549 and 1.555, the

26 refractive index increasing progressively from the surface toward theinterior.

EXAMPLE 12 A glass fibre comprising a glass-fibre core composed of 10.2percent of T1 0, 176 percent of Na- O, 9.0 percent of B 0 and 63.2percent of SiO (all in mol percent) and having a 0.2 mm. diameter and asurface covering layer composed of 10.2 percent of K 0, 17.6 percent ofNa O, 9.0 percent of B 0 and 63.2 percent of v EXAMPLE 13 A glass fibrecomprising a glass-fibre core composed of 4.2 percent of Tl O,12.5percent of Na O, 33.3 percent of MgO, and 50.0 percent of P 0 (all inmol percent) and having a diameter of 0.3 mm. and a surface coveringlayer having the same composition as the core and having a thickness of0.15 mm. was prepared. The refractive index of the core was 1.53, whilethat of the covering layer glass was 1.51. This glass fibre was heatedto a temperature of 400C and thus maintained for 72 hours.

The refractive index within the glass fibre thus treated was found toexhibit a distribution such that, within a circle of 0.2 mm. radius fromthe fibre centre as viewed in cross sections, the refractive index n ata distance r (cm.) from the centre satisfied the following equation.

n 1.53 (l al' where a 60 cm? EXAMPLE 14 A glass rod composed of 56percent of SiO 14 percent of Na O, 20 percent of T1 0, and 10 percent ofPbO and having a 1 mm. diameter was prepared and steeped for 24 hours ata temperature of 500C in a KNO bath containing 0.2 percent of T1 N0 Theglass rod thus treated was found to have a refractive index N at itscentre of 1.56 and at its surface of 1.53 and a refractive indexdistribution wherein the following equation was satisfied.

N =N,, (1 m where:

N is the refractive index of the central part and This glass rod was cutto a speciric length, and its two ends were ground perpendicularly tothe centre axis thereof, whereupon the glass rod was found to exhibit anexcellent lens effect.

EXAMPLE 15 A glass rod composed of 56 percent of SiO 14 percent of Na O,and 30 percent of T1 0 and having a 1 mm. diameter and 200 mm. lengthwas prepared and steeped for 15 hours at 510C in a KNO bath containing1.7 percent of T1 N0 As a result, a glass rod having a refractive indexdistribution such that, in the cross section of the rod, the indexincreased progressively from the periphery 27 toward the centre, and aparabolic distribution symmetrical about the centre was indicated wasobtained. This glass and was cut to a specific length, and the two endsthereof were ground perpendicularly to the centre axis, whereupon theglass rod was found to have an excellent lens effect.

EXAMPLE 16 A glass fibre having the same composition as the rod inExample 14 and having a 0.5 mm. diameter and a 500 mm. length was formedand steeped for 22 hours at 475C in a KNO bath containing 0.5 percent ofT1NO As a result, the glass fibre was found to have a refractive indexof parabolic distribution symmetrical about the centre of the fibre asviewed in cross section, the index increasing progressively from theperiphery toward the centre.

The two ends of this glass fibre were ground flat in planesperpendicular to the centreline of the fibre, and a gas laser beam wasintroduced in a mode-matched state into one end of the fibre, whereuponthe beam was conducted with high efficiency along the centre axis of thefibre without any impairment of the laser polarisability.

EXAMPLE 17 A glass core rod composed of 69.5 percent of SiO 25.5 percentof CdO, and 5.0 percent (all in mol percent) of T1 and a glass coveringtube composed of 74.8 percent of SiO 20.2 percent of BaO, and 5.0percent of K 0 and having an inner diameter somewhat greater than thediameter of the core rod were prepared.

Then, as indicated in FIG. 13, the covering tube 132 and the core rod131, maintained in concentric relationship, were gradually passed fromabove through the heating device 133, in which the core rod 131 andcovering tube 132 fused together. The rod and tube thus fusedtogetherwere then passed between rolls 134, whereupon a covered glassrod was obtained.

This rod had a core of 0.10 cm. diameter and a covering layer of 0.20cm. outer diameter. The refractive index of the core was 1.645, whilethat of the covering layer was 1.567.

This covered rod was heat treated for 24 hours in an electric furnace at480C. The components of the rod glass and the covering layer glass whichcannot diffuse at 480C are SiO -CdO and SiO -BaO, respectively, theconcentrations of which in this instance were both 95 mol percent.Furthermore, in the case where these components form glassesindependently, the resulting glasses (that is, a glass composed of 73.1mol percent of SiO and 26.9 mol percent of CdO and a glass composed of78.8 mol percent of SiO and 21 .2 mol percent of BaO) both have arefractive index of 1.565.

The thallium ions within the core glass and the potassium ions withinthe covering layer glass diffuse respectively within the covering layerand the core during heat treatment. The refractive index distributionwithin the covered glass rod after heat treatment in this instance wasas indicated in FIG. 18. In the regions in the vicinity of the glass rodcentre, the refractive index decreased substantially in proportion tothe square of the distance from the centre, but in the peripheralregions, particularly in parts at distances exceeding 0.075 cm. from thecentre, the refractive index deviated greatly from the quadratic curve.

Accordingly, the peripheral part of this glass rod was removed bydissolving with hydrofluoric acid to produce a glass rod of 0.15-cm.diameter.

The refractive index distribution Within the glass rod thus produced wasfound to be representable substantially by the following equation.

N N,, (l mr"), where:

N is the refractive index at the rod cross sectional centre and wasequal to 1.640;

r is the distance from the centre; and

a is a positive constant and was equal to 8.3 cm.

Furthermore, the refractive index was found to vary in a continuousmanner on both sides of the original interface between the core glassand covering layer glass.

A piece of 0.5 cm. length was cut from this glass rod and both cutsurfaces were ground perpendicularly to the centre axis, whereupon aconvex lens having a focal distance of 0.16 cm. was obtained.

EXAMPLE 18 A covered glass rod obtained by fusion in the heating devicein the same manner as set forth in Example 17 was steeped for 48 hoursin a sodium nitrate bath maintained at 430C.

The glass rod thus treated was found to have a refractive indexdistribution which conformed to a quadratic curve not only in thecentral part thereof but also throughout the peripheral parts. Thereason for this can be considered to be that, in addition to the mutualdiffusion of the thallium ions within the core glass and the potassiumions within the covering layer glass, mutual diffusion principally ofthe potassium ions within the covering layer glass and the sodium ionswithin the bath occurred, whereby the refractive index in the vicinityof the outer side of the covering layer decreased.

The internal refractive index N of this rod was found to berepresentable substantially by the following equation.

N=N (1 ar )v where:

N, is the refractive index at the cross sectional centre of the rod andwas equal to 1.640 in this instance;

r is the distance from the centre; and

a is a positive constant and was equal to 4.3 cm? A piece of 0.5 cm.length of this glass rod was cut out, and the two cut surfaces wereground perpendicularly to the centre axis, whereupon a convex lens of afocal distance of 0.21 cm. was obtained.

EXAMPLE 19 Core glass cullet composed of 80.3 percent of SiO 12.7percent of PhD, and 7.0 percent of Tl ,an intermediate layer glasscullet composed of 77.7 percent of SiO 12.3 percent of PbO, 4.0 percent,of T1 0, and 6.0 percent of K 0, and an outer layer glass culletcomposed of 69.3 percent of SiO 17.7 percent of BaO, and 13.0 percent ofK 0 (all in mol percent) were prepared and fabricated into atriple-lamination glass rod by means of a triple pot, as shown in FIG.19.

More specifically, the above specified core, intermediate layer, andouter layer glass cullets 238, 239, and 240 in molten state were placerespectively in inner pot 235, intermediate pot 236, and outer pot 237of the triple pot provided respectively with electrodes 241, 242, and243 for electric heating. The three molten glasses thus placed flowedout through respective nozzles at the bottoms of the pots and, beingguided by guide rol-

1. A METHOD FOR PRODUCING A GLASS FIBROUS STRUCTURE FOR CONDUCTING LIGHTALONG THE CENTER AXIS THEREOF, CHARACTERIZED BY HAVING A REFRACTIVEINDEX DISTRIBUTION WITHIN SAID STRUCTURE WHEREIN A REFRACTIVE INDEXDECREASE CONTINUOUSLY IN A DIRECTION RADIALLY AND OUTWARDLY FROM SAIDCENTER AXIS TOWARD THE OUTER SURFACE OF SAID STRUCTURE, SAID METHODCOMPRISING: PROVIDING A GLASS FIBER HAVING A CONSTANT REFRACTIVE INDEXAND A GENERALLY CIRCULAR CROSS-SECTION AND CONTAINING THEREIN A GLASSFORMING OXIDE AND A FIRST MONOVALENT CATION CONSTITUTING A GLASSMODIFYING OXIDE, CONTACTING SAID GLASS FIBER WITH A SOURCE OF SECONDMONOVALENT CATION ALL ALONG THE OUTER SURFACE OF THE GLASS FIBER AT AHIGH TEMPERATURE AND FOR A SPECIFIC TIME TO THEREBY CAUSE MIGRATION OFSAID FIRST TEMPERATURE TO THEREBY CAUSE INTERIOR OF SAID GLASS FIBER ATSAID TEMPERATURE TO THEREBY CAUSE SUBSTITUTION OF SAID SECOND CATION FORSAID FIRST CATION IN THE GLASS OF SAID GLASS BIBER SO AS TO DECREASE THEREFRACTIVE INDEX OF THE GLASS FIBER IN A RADIALLY OUTWARD DIRECTION FROMSAID CENTER AXIS, AND CONTROLLING SAID SPECIFIC TIME IN SUCH A MANNERTHAT SAID SECOND CATION IN SAID SOURCE MIGRATES INTO SAID GLASS FIBERTHROUGH SAID OUTER SURFACE OF SAID GLASS FIBER TO THEREBY CAUSESUBSTITUTION OF SAID SECOND CATION FOR SAID FIRST AND SECOND CATIONSWITHIN THE GLASS FIBER, SAID GRADIENTS CORRESPONDING RESPECTIVELY TOCONTINUOUS INCREASE AND DECREASE OF SAID FIRST AND SECOND CATIONS FROMSAID SURFACE TOWARD SAID CENTER AXIS OF SAID STRUCTURE TO THEREBYESTABLISH SAID REFRACTIVE INDEX DISTRIBUTION BY SAID CONCENTRATIONGRADIENTS OF SAID FIRST AND SECOND CATIONS.
 2. A method for producing aglass fibrous structure as claimed in claim 1, in which said first andsecond cations are respectively thallium ion and alkali metal ion.
 3. Amethod for producing a glass fibrous structure as claimed in claiM 1, inwhich said specific time is controlled so as to enable said secondcation to reach at least said center axis to thereby cause decrease ofthe refractive index in a radial and outward direction and substantiallyin proportion to the square of the distance from the center axis atleast in the vicinity of the center axis.
 4. A method for producing aglass fibrous structure as claimed in claim 1, in which said source ofsecond monovalent cation comprises a glass containing therein saidsecond cation; and wherein said contacting comprises fusing said glassonto the surface of said glass fiber.
 5. A method for producing a glassfibrous structure as claimed in claim 1, in which said source of secondmonovalent cation comprises a salt containing said second cation.
 6. Amethod for producing a glass fibrous structure as claimed in claim 1, inwhich said temperature is higher than the glass strain point and lowerthan the glass softening point of said glass fiber.
 7. A method forproducing a glass fibrous structure as claimed in claim 2, in which saidglass fiber consists of a silicate glass containing thallium ion.
 8. Amethod for producing a glass fibrous structure as claimed in claim 1, inwhich said glass fiber is, after said contacting, heated and drawn inthe longitudinal direction thereof to reduce the cross-sectional size ofsaid glass fiber.
 9. A METHOD FOR PRODUCING A GLASS FIBROUS STRUCTUREFOR CONDUCTING LIGHT ALONG THE CENTER AXIS THEREOF, CHARACTERIZED BYHAVING A REFRACTIVE INDEX DISTRIBUTION WITHIN SAID STRUCTURE WHEREIN THEREFRACTIVE INDEX DECREASE CONTINUOUSLY IN A DIRECTION REDIALLY NDOUTWARDLY FROM SAID CENTER AXIS TOWARD THE OUTER SURFACE OF SAIDSTRUCTURE, SAID METHOD COMPRISING: PROVIDING A GLASS FIBER HAVING ACONSTANT REFRACTIVE INDEX AND A GENERALLY CIRUCLAR CROSS-SECTION ANDCONTAINING THEREIN A GLASS FORMING OXIDE AND A FIRST CATION CONSTITUTINGA GLASS MODIFYING OXIDE, CONTACTING SAID GLASS FIBER WITH A GLASSCONTAINING SECOND CATION ALL ALONG THE OUTER SURFACE OF THE GLASS FIBERAT A HIGHER TEMPERATURE AND FOR A SPECIFIC TIME TO THEREBY CAUSEMIGRATION OF SAID FIRST AND SECOND CATIONS IN THE INTERIOR OF SAID GLASSFIBER AT SAID TEMPERATURE TO THEREBY CAUSE SUBSTITUTION OF SAID SECONDCATION FOR SAID FIRST CATION CONTAINED IN THE GLASS OF SAID GLASS FIBERSO AS TO DECREASE THE REFRACTIVE INDEX OF SAID GLASS FIBER IN A RADIALLYOUTWARD DIRECTION FROM SAID CENTER AXIS, SAID CONTRACTING BEING CARRIEDOUT BY FUSING SAID GLASS ONTO THE SURFACE OF SAID GLASS FIBER, ANDCONTROLLING SAID SPECIFIC TIME IN SUCH A MANNER THAT SAID SECOND CATIONIN SAID SOURCE MIGRATES INTO SAID GLASS FIBER THROUGH SAID OUTER SURFACEOF THE GLASS FIBER TO THEREBY CAUSE SUBSTITUTION OF SAID SECOND CATIONFOR SAID FIRST CATIONS SO AS TO FORM CONCENTRATION GRADIENTS OF SAIDFIRST AND SECOND CATIONS WITHIN THE GLASS FIBER, SAID GRADIENTSCORRESPONDING RESPECTIVELY TO CONTINUOUS INCREASE AND DECREASE OF SAIDFIRST AND SECOND CATIONS FROM SAID SURFACE TOWARD SAID CENTER AXIS OFSAID STRUCTURE TO THEREBY ESTABLISH SAID REFRACTIVE INDEX DISTRIBUTIONBY SAID CONCENTRATION GRADIENTS OF SAID FIRST AND SECOND CATIONS.
 10. Amethod for producing a glass fibrous structure for conducting lightalong the center axis thereof, characterized by having a refractiveindex distribution within said structure wherein the refractive indexdecreases continuously in a direction radially and outwardly from saidcenter axis toward the outer surface of said structure, said methodcomprising: providing a glass fiber having a constant refractive indexand a generally circular cross-section and containing therein a glassforming oxide and a first cation constituting a glass modifying oxide;contacting said glass fiber with a source of a second cation all alongthe outer surface of the glass fiber at a high temperature and for aspecific time to thereby cause migration of said first and secondcations in the interior of said glass fiber at said temperature tothereby cause substitution of said seCond cation for said first cationin the glass of the glass fiber so as to decrease the refractive indexof said glass fiber in a radially outward direction from said centeraxis; and controlling said specific time in such a manner that saidsecond cation in said source migrates into said glass fiber through saidouter surface of the glass fiber thus causing substitution of saidsecond cation for said first cation to thereby form concentrationgradients of said first and second cations within the glass fiber, saidgradients corresponding respectively to continuous increase and decreaseof said first and second cations from said surface toward said centeraxis of the structure to thereby establish said refractive indexdistribution by said concentration gradients of said first and secondcations, said specific time being controlled so as to enable said secondcation to reach at least said center axis to thereby cause decrease ofthe refractive index in a radial and outward direction and substantiallyin proportion to the square of the distance from the center axis atleast in the vicinity of the center axis.